The present disclosure relates to an image coding device, an image decoding device, methods thereof, and programs, and particularly to enabling improvements of subjective image quality and coding efficiency.
In related art, still images and moving images have large amounts of data, and are thus generally coded at a time of transmission or at a time of recording onto a medium. A coding system such as an H.264/MPEG (motion picture expert group)-4 AVC (advanced video coding) (hereinafter referred to as H.264/AVC) system or the like performs a discrete cosine transform (hereinafter referred to as a DCT)/inverse discrete cosine transform (hereinafter referred to as an IDCT). The DCT/IDCT is realized by performing a one-dimensional DCT/IDCT twice in a horizontal direction and a vertical direction. On the other hand, when prediction errors of intra-frame prediction (intra prediction) include an edge, energy can be concentrated more by performing a DCT in a direction along the edge rather than performing a DCT in the horizontal direction and the vertical direction.
For example, Japanese Patent Laid-Open No. 2009-272727 determines a direction of performing an orthogonal transform according to a mode of intra-frame prediction when intra-frame prediction as in the H.264/AVC system using spatial correlation is performed because of a strong possibility that a prediction direction and the direction of prediction errors are the same. By thus determining a direction of performing an orthogonal transform, higher energy concentration is achieved, and coding efficiency is improved.